In general, to a flow path for fluid such as air, water, oil, etc., a switching valve for allowing or blocking the flow of the fluid is provided. Such switching valve has a first port and a second port for fluid flow, a valve seat for connecting the first port and the second port is provided in the fluid path, and a valve member for opening and closing the fluid path by contacting with and separating from the valve seat is provided. The valve member is connected to a piston through a rod, a pressure operating chamber is provided on the valve member side of the piston, and a coil spring is provided on the opposite side. The piston, by supplying pressure fluid from a pilot valve to the pressure operating chamber, moves the valve member in a direction separating the valve seat against spring force of the coil spring for return, and while discharging the pressure fluid, moves the valve member in a direction pressing and contacting to the valve seat by using the spring force of the coil spring Japanese Unexamined Patent Application Publication No. 2001-193846).
Such pilot-type two-port valve which drives the piston by using the pressure fluid and the coil spring quickly operates, that is, the speed to drive the piston to open and close the fluid path by separating the valve member from the valve seat and bringing the valve member in contact with the valve seat is fast. Accordingly, the pilot-type two-port valve is preferable to the valve for gas and low-pressure liquid.
However, particularly in a liquid flow path of high-pressure liquid such as coolant, due to rapid increase of valve-outlet pressure at the time of valve opening, sometimes foreign material such as chips which are flowing in the flow path intrude into a seal part in an insertion hole in a plate through which a rod passes. In such a case, the sealing material in the seal part may be damaged and the sealing performance at the seal part may be decreased. Then, the liquid which is flowing in the liquid flow path may flow into a valve driving mechanism and the valve function of the switching valve can be lost.
Such a problem occurs to not only the normally closed or opened single-acting valve which uses the coil spring for return but also occurs to a double-acting valve.